Acoustic resonators based on the piezoelectric properties of certain materials are ubiquitous in many technical fields. For instance, film bulk acoustic resonators (FBARs) are used in communications devices for electrical filters, and in electrical devices for voltage transformer, to name merely a few applications. FBAR devices generate both longitudinal waves and lateral (or transverse) waves when stimulated by an applied time-varying electric field. Additionally, higher order harmonic mixing products may be generated. As is known, the lateral modes and the higher order harmonic mixing products are often not desired and can have a deleterious impact on the functionality of the FBAR-based device.
One type of electrical filter application of FBARs is a passband filter used in duplex communications. As is known by one of ordinary skill in the art, duplex filters are used to provide isolation between a transmit function of a duplexer and a receive function of the duplexer. Thus, two filters are provided, and each is designed to function within certain specifications that include prescribed pass-band transmission, out-of-band attenuation and roll-off, to name a few common specifications.
One particular specification is the so-called third generation (3G) specification proffered under the Universal Mobile Telecommunications System (UMTS). The 3G specification includes a quad-band requirement that allows mobile communication devices (e.g., cellular phones, personal digital assistants (PDAs), and portable computers) to more easily roam between different countries that peg the allowed transmission frequency at different values or to allow a better coverage in the same country.
One difficulty in addressing signal filtering (e.g., duplex filtering) in the 3G specification are higher order harmonics generated in the piezoelectric material due to non-linear properties of the piezoelectric material. These higher order harmonics produce higher order mode mixing products, which can result from both mixing products of longitudinal mode and lateral mode mixing products. Unfortunately, known acoustic filters generate these mixing products at frequencies and power levels not allowed by the 3G specification.
Certain attempts have been made to reduce the mixing products. One attempt provides separate FBARs connected to in an effort to cancel certain higher order modes. However, there are drawbacks to this known attempt. Most notably, the cancellation is poor at certain frequency ranges where parasitic lateral modes are found. Moreover, the quality (Q) factor in these separate FBAR configurations is degraded compared to known FBARs. The degradation in the Q factor is manifest in a degradation in the insertion loss in the passband of the separate FBAR devices.
Thus, the performance of a filter based on such a device is often unacceptable. Moreover, the multiple separate FBAR devices result in increased chip area for the filter. Not only does this increase the size of the filter, but also results in an increase in the cost of fabrication of the filter. Both increased chip size and increased manufacturing costs are undesired.
There is a need, therefore, for an acoustic resonator and a filter that overcomes at least the shortcoming of known resonators and filters discussed above.